Metallic glasses, unlike conventional crystalline alloys, have amorphous or disordered atomic-scale structures that give rise to unique chemical, mechanical, and rheological properties. Owing to their atomic structure, metallic glasses generally exhibit better corrosion resistance than typical crystalline alloys, higher hardness, strength, and elasticity, and are able to soften and flow when relaxed above their glass transition temperatures (Tg), a characteristic that allows for considerable processing capability. Previously, metallic glasses were only capable of being produced in sub-millimeter dimensions (thin ribbons, sheets, wires, or powders) due to the need for rapid cooling from the liquid state to avoid crystallization. However, recent developments in bulk glass-forming alloys have enabled the production of metallic systems with very sluggish crystallization kinetics able to form glasses in dimensions as large as several centimeters. These developments have permitted the introduction of metallic glasses in many engineering applications where their unique chemical and mechanical properties, including good corrosion resistance, high strength and hardness, and large elastic elongation, are desirable.
The most robust glass-forming metallic system to date is a Pd—Ni—Cu—P alloy, which is capable of forming amorphous parts with thicknesses as large as seven centimeters. The ability to produce metallic glass ingots of such increased thickness has aroused interest in many applications. However, due to the prohibitively high cost of Pd (a noble metal) most of these applications remain out of reach. Applications for which the high cost of noble metals (such as Pd) is not considered as prohibitive include jewelry and biomedical applications (such as orthopedic and dental/orthodontic applications). Interestingly, the noble-metal character of Pd makes Pd-based glasses particularly attractive for such applications. However, the only Pd-based metallic glasses known to achieve dimensions of a few millimeters or more contain either or both Ni and Cu in the alloy composition. Indeed, the glass-forming ability of metals in general is widely known and recognized to be heavily dependent on the inclusion of Ni and/or Cu in the alloy, and it is the inclusion of these metals that enabled the development of such robust metallic glass formers. As such, the inclusion of Ni and Cu is widely accepted as necessary to the formation of glass-forming alloys, and skilled artisans in the field would have no expectation of success in creating a good glass-forming alloy without including at least one of these metals.
While the inclusion of Ni and Cu in metallic glasses is generally suitable and acceptable when the glasses are used for engineering applications, these metals are highly cytotoxic, making metallic glasses including these metals ill suited for biomedical applications. In particular, Ni and Cu are highly electronegative, allowing them to exist as free radicals in the blood stream. Such free radicals are notorious triggers for severe adverse biological reactions in the body. Consequently, Ni and Cu are widely understood and regarded as non-biocompatible, and any metallic glasses including these metals are similarly understood to be non-biocompatible. As the glass-forming ability of amorphous metal alloys is strongly dependent on the inclusion of Ni and/or Cu, development of Pd-based metallic glasses suitable for use in biomedical applications has proved extremely challenging, and no suitable such metallic glass has yet been achieved.